I do think Robyn was Whitney's soulmate and the one holding her together as much as she could. You could tell in that Dateline interview how much Robyn still loved her and how much it stung to be cut out of Whitney's life when she was.
I don't think anyone knew Whitney the way Robyn did, and despite Cissy's (and others) objections, Whitney brought Robyn along before and after she experienced success. Probably because she trusted her deeply and knew no one would look out for her the same way. And really, we've already seen that no one else in Whitney's circle had the same kind of respect for her - especially her own damn family.